This invention relates to a method and apparatus for detecting and measuring repetitive patterns of nonuniformities in film or sheet materials such as paper, plastics or foil products using nuclear gauging devices.
Nuclear (beta ray) gauges have been used since the early 1950's for the measurement of sheet or film products such as paper or plastic or metal foils. Such measurements are of great value to the manufacturing industry since the gauges can be used to measure and process control while the product is being fabricated.
Nuclear gauges operate by detecting the nuclear radiation emitted by a source after passage through the material to be measured. Since radiation is partially absorbed by the material to be measured, it is possible to relate the thickness (or areal density) of the material to the degree to which the radiation has been absorbed or attenuated during passage. The thicker the material, the greater is the attenuation of the transmitted radiation.
It is also common to make such measurements by means of measuring radiation that has been scattered or reflected from the material to be measured, rather than transmitted. Using this technique, the detected radiation increases with increasing thicknesses. In either case, the change in the detected radiation with change in the thickness of the material being measured is usually approximated by an exponential function.
Nuclear radiation is radiation emitted when an atom disintegrates spontaneously, ejecting either a photon (gamma or X-rays) or a particle (alpha, beta or neutrons). Often, more than one form of radiation is emitted from any given source.
In all cases, the disintegration process (radioactivity) is purely a random phenomenon, it is not constant, but is statistical in nature. In fact, nuclear radiation follows a Poisson distribution, and when detected with sufficient sensitivity, it is seen to be quite random or noisy in character. The radiation is emitted in discrete pulses, irrespective of the particular physical form (alpha, beta, gamma or neutron). For a Poisson distribution, the mean statistical deviation is the square root of the number of events, and is quite easy to calculate when the detected transmitted (reflected) radiation is known. The normalized statistical deviation, or practically speaking, the noise observed at the detector, is thus related to the reciprocal square root of the source strength, as well as the reciprocal of the square root of the duration of measurement. Or, more simply put, to the reciprocal square root of the radiation events (disintegration) actually measured.
Since nuclear radiation is generally considered to be dangerous to one's health, it is not surprising to find that radiation levels in most nuclear gauging applications are so weak as to show well pronounced noise. In order to make sensitive measurements, it is therefore necessary to take the measurements over appreciable lengths of time, or in other words, to employ long measuring time constants. Quite evidently, a compromise is made between measurement sensitivity and measurement time constant or the response rate of a nuclear gauge.
Beta gauges, as used today, have typical time constants of a tenth of a second. To obtain measurements of high resolution, the gauging signals may be subjected to averaging with equivalent time constants of up to ten seconds or more. For relatively coarse measurements, time constants as short as 10 milliseconds have been used.
Products, such as paper, are made on high speed machinery, and production speeds of 3000 feet per minute are not uncommon, a line speed of 50 feet per second or 0.6 inches per millisecond. To make real time measurements using nuclear gauges with any degree of precision over distances of inches or less is therefore apparently not possible, at least not by conventional techniques.
The gauge response rate is also determined by the response rate of the detectors used and to some degree, the associated electronics. The detector predominantly used today is the ionization chamber, which is inherently a slow device. Less commonly used is the detector comprising a scintillator - photomultiplier combination. This type of detector can be operated in either of two modes. The simpler mode is to use the detector in an analog or DC current mode. The second mode is to use the detector as an event or pulse counter. The earliest beta gauges was an event counting scintillator photomultiplier. That device proved totally impractical at the time (early 1950's) due to the lack of fast electronic counters. The scintillation counter was replaced in the 1950's by the ionization chamber, and later by the analog scintillation detector.
The scintillation detector, in either the DC or in the event counting mode, is extremely fast when compared to the ionization chamber, and unlike the ionization chamber may, in principle, be used for fast gauging.